1. Field of the Invention
The present invention relates to a hybrid electric vehicle and a powertrain for such a vehicle.
2. Background Art
With the ever increasing need to produce vehicles that are more fuel efficient, hybrid electric vehicles (HEV's) have provided an improvement in fuel economy over many conventional vehicles that utilize only an internal combustion engine to drive the vehicle. One of the primary advantages of an HEV is that it allows the vehicle to be powered by one or more electric machines under certain operating conditions. For example, if the speed of the vehicle is relatively moderate, and the battery or other electrical power source is sufficiently charged, the engine may be shut off, and the vehicle powered exclusively by the electric motors. As operating conditions change, the engine may be started to provide additional power, and if needed, operate a generator to charge the battery.
Various arrangements of internal combustion engines and electric machines—i.e., generators and motors—have been utilized in HEV's. For example, series arrangements, parallel arrangements, and power-split arrangements provide different ways for the engine and electric machines to interact to output torque to drive the vehicle. In one example of a power-split arrangement, an engine and an electric machine are both operatively connected to a planetary gear set, the output of which provides torque to the vehicle drive wheels. One or more additional electric machines can be used to provide additional torque to the drive wheels, charge an energy storage device, such as a battery, provide electrical energy directly to another electric machine, or some combination thereof.
In the power-split arrangement, power output from the engine flows through the planetary gear set, where a portion of the power is applied to the electric machine connected to the planetary gear set, and another portion of the engine power is transferred to the vehicle drive wheels. In some power split arrangements, a second electric machine—e.g., a motor not connected to the planetary gear set—is used to drive the vehicle in reverse. This is a relatively simple operation, in that an electric motor can be operated to output torque in either of two opposite rotational directions. This eliminates the need for complex gear and clutch systems that may otherwise be required to drive the vehicle in reverse. Because internal combustion engines are configured to provide power in one direction only, many HEV's rely exclusively on one or more electric machines to drive the vehicle in reverse.
Although electric motors provide a convenient way to drive an HEV in reverse, additional torque may be desired in some situations. For example, in off-road situations, or where traction is bad, an operator may desire a greater reverse torque than a motor can generally provide. In such situations, it would be desirable to use the engine in the HEV to drive the vehicle in reverse.
One example of an HEV that is configured to allow the engine to output torque to drive the vehicle in reverse is described in U.S. Pat. No. 5,847,469 issued to Tabata et al. on Dec. 8, 1998. Tabata et al. describes an HEV that allows either the engine or the electric motor to drive the vehicle in reverse. In order to accomplish this, the vehicle described in Tabata et al. includes an elaborate series of gears, clutches, and brakes to manipulate the torque output from the engine, such that it is capable of driving the vehicle in reverse. The system described in Tabata et al. contains many elements, each of which has to interact with one or more other elements, thereby adding complexity, weight, and additional cost to the vehicle. Therefore, it would be desirable to have an HEV, and a powertrain for an HEV, that were configured to allow torque from the engine to drive the vehicle in reverse, without adding a large number of components that would undesirably increase the complexity and cost of the vehicle, and reduce the efficiency of its operation.